The information recording media of the prior art include a phase-change type information recording medium wherein a recording layer is formed from a phase-change material. Among the phase-change type information recording media, a medium that optically records, erases, overwrites and reproduces information by using laser beam is called an optical phase-change information recording medium (which may hereinafter be referred to simply as optical recording medium). Information is recorded on the optical recording medium by irradiating a phase transition material with a laser beam so that heat generated thereby causes the phase-change material to change between, for example, crystal phase and amorphous phase. The recorded information is reproduced by detecting the difference in reflectivity between the crystal phase and the amorphous phase.
The optical recording media also include a rewritable optical recording medium that allows erasure and overwriting of information, in which the initial state of the recording layer is generally crystal phase. To record information on this medium, it is irradiated with a laser beam of high power so as to melt the recording layer and is then cooled down quickly, thereby turning the portion of the recording layer that has been irradiated with the laser into amorphous phase. In contrast, to erase information from this medium, it is irradiated with a laser beam having an output power lower than that for recording, so as to raise the temperature of the recording layer and then gradually cool it, thereby turning the portion of the recording layer irradiated with the laser into crystal phase. It is made possible to record new information while erasing the information that is already recorded, that is, to overwrite the information, by irradiating the recording layer with a laser beam that has been power-modulated between a high power level and a low power level.
In order to erase or overwrite information faster, it is necessary to cause the transition from amorphous phase to crystal phase to take place in a shorter period of time. In other words, achieving higher erasure performance with a rewritable optical recording medium requires it to the recording layer to be formed from a phase-change material that has a higher crystallization rate.
Such a rewritable optical recording medium is constituted from, in addition to the recording layer, dielectric layers, a reflective layer, an interface layer, etc, formed on a substrate. The dielectric layer may be provided for the purpose of preventing the recording layer from evaporating and the substrate from thermally deforming during repetitive recording operations, and for making it possible for the recording layer to efficiently absorb light and undergo optical change through optical interference effect. The dielectric layers are provided on both sides of the recording layer. The reflective layer is provided for the purpose of efficiently utilizing the applied laser beam, and increasing the cooling rate so as to enable the recording layer to readily undergo transition to amorphous phase. The reflective layer is usually disposed behind the recording layer when viewed from the side of the laser source such that the reflective layer and the recording layer sandwich the dielectric layer. The interface layer is disposed, as required, between the recording layer and the dielectric layer, for the purpose of preventing atoms and molecules from diffusing between the recording layer and the dielectric layer.
Known phase-change materials include GeTe—Sb2Te3, GeTe—Bi2Te3 and GeTe—SnTe. In particular, GeTe—Bi2Te3 has a high crystallization rate and therefore can be used to form the recording layer of a rewritable optical recording medium and achieve excellent erasability (refer to Patent Document 1).
For the purpose of increasing the recording capacity of the rewritable optical recording medium, a method of providing two information layers in a single optical recording medium has been proposed (refer to Patent Document 2). Information can be recorded in the two information layers by means of laser beam incident on one side of the medium to each of the two information layers. Reproduction is also carried out similarly. Accordingly, the optical recording medium that has two information layers may have a recording capacity almost twice that of an optical recording medium that has a single information layer.
In the rewritable optical recording medium having two information layers, the information is recorded and reproduced on and from the information layer that is located further from the laser source (hereinafter referred to as a first information layer) by means of a laser beam that has passed an information layer that is located nearer to the laser source (hereinafter referred to as a second information layer). Therefore the second information layer preferably has a transmittance that is as high as possible. The phase-change materials generally have a high extinction coefficient, and therefore the recording layer of the second information layer is preferably thinner in order to increase the transmittance of the recording layer.
Also in order to obtain an optical recording medium that has more information layers, for example three or four information layers, so as to increase the recording capacity, it is required to further increase the transmittance of the information layer located on the laser source side (a third information layer or a fourth information layer), and therefore a thickness of the recording layer must be further decreased.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 63-225935    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No 2000-36130